In automotive vehicles equipped at one axle with disk brakes and at the other axle with brake shoes and drums, a check valve is required for the brake-shoe pressure lines in order to maintain a minimum pressure therein. The check valve includes a disk held by a spring against a sealing surface, as illustrated on page 27, FIGS. 7 and 8, of the German-language "Brake Handbook" ("Bremshandbuch"), 5th edition, 1977, published by the Bartsch Verlag of Ottobrunn, West Germany.
Such check valves are operable if the hydraulic system in which they are disposed contains only fluid, i.e. is substantially devoid of air. Generally, during a charging or filling operation, brake fluid is permitted to flow through the hydraulic system for such a time as to ensure removal of practically all the air from the cylinders and pressure-transmission conduits. Disadvantages of this method of air discharge are the amount of time and the quantity of brake fluid involved.
Another method of charging an automotive braking system with force-transmitting fluid comprises the steps of depressurizing the system to approximately 5 millibars absolute pressure and then filling the system with brake fluid subjected to 2.5 bars absolute pressure. Such a procedure for charging a braking system requires less time and a smaller quantity of brake fluid than methods merely using gravity to remove air and force brake fluid into all branches of the system. However, conventional valves pose a problem to this otherwise more efficient method of charging automotive braking systems. The spring-loaded sealing disks of conventional valves prevent peripheral parts of the braking system from being evacuated. A connection of a vacuum pump to the master cylinder results only in the evacuation of that portion of the system extending from the master cylinder to the check valves. In the wheel cylinders and the conduits between the check valves and the wheels, air remains at a normal pressure level during the depressurization of the master cylinder, thereby hindering a complete filling of the braking system with fluid.